Structure of the magnetopause for low Mach number and
strongly northward interplanetary magnetic field

Abstract. We use ISEE magnetic field and plasma data to examine dayside
magnetopause crossings under conditions of low Mach number and strongly northward
interplanetary magnetic field (IMF). When the solar wind Mach number is low, the IMF
strength and magnetosheath field strength are large, and we expect the effects of magnetic
reconnection to be the strongest. When the IMF is strongly northward, we find that the
location of the magnetopause boundary layer is very stationary in the space, and we
observe many features that are common for both typical and low Mach numbers.
However, under low Mach number conditions, we have observed some features that would
be expected for cusp reconnection. The boundary layer near the subsolar region contains
heated magnetosheath plasma with little hot magnetospheric component that has clearly
entered the magnetosphere elsewhere. At least some of the structures present in the
boundary layer are impulsive. Inside the boundary layer there is also clear evidence of
accelerated flow from the cusp region for strongly northward IMF at low Mach number.
Reconnection beyond the cusp can explain the observed field, plasma, and flow signatures.
Therefore at low Mach number, reconnection is important in the formation of the
boundary layer for northward IMF.

1. Introduction

The magnetopause is the interface between the shocked
solar wind in the magnetosheath and the Earth's
magnetosphere. It is a complex plasma boundary consisting
of both field and plasma transitions. The change of the
magnetic field across the magnetopause is associated with a
current layer, which is generally taken to be carried by
deflected solar wind particles [Chapman and Ferraro, 1931].
Much of the incoming plasma flows tangential to this
boundary at the magnetopause. On the sunward side of this
boundary, the plasmas are dense and cold. On the earthward
side of this boundary the plasmas are hot and tenuous. The
region immediately earthward of the magnetopause was first
studied more than 20 years ago and at low latitude was
called low-latitude boundary layer (LLBL) [Hones et al.,
1972; Akasofu et al., 1973]. Early observations revealed that
the plasma inside this boundary layer has properties
intermediate between those of the magnetosheath and the
magnetosphere [Eastman et al., 1976; Haerendel et al.,
1978; Eastman and Hones, 1979]. However, the observations
from IMP 6 and Heos 2 differed in one important plasma
property that led to two distinct mechanisms for the
formation of the LLBL. In the IMP 6 data the density fell off
gradually to the magnetospheric background level across the
LLBL, and a local diffusion process was proposed [Eastman
et al., 1976; Eastman and Hones, 1979]. However, in the
Heos 2 data the density was roughly constant throughout the
LLBL, and a mechanism of nonlocal entry from the cusp
and/or heating of cold magnetospheric plasma were suggested
[Haerendel et al., 1978]. The discrepancy in the two data
sets may be due to the low time resolution of Heos 2 data
[Eastman and Hones, 1979] or it may be due to the different
spatial coverage of the dayside magnetopause by Heos 2 and
IMP 6 [Paschmann et al., 1978].

ISEE and AMPTE spacecraft have provided both
significantly more observations of the Earth's magnetosphere
and data at higher time resolution than available previously.
The structure of the magnetopause boundary layer and its
plasma properties have been under active investigation using
these data and data from other spacecraft [e.g., Paschmann
et al., 1978, 1990; Sckopke et al., 1981; Ogilvie et al., 1984;
Mitchell et al., 1987; Ogilvie and Fitzenreiter, 1989; Gosling
et al., 1990; Song et al., 1990b; Takahashi et al., 1991]. It
has long been recognized that the physical processes that
govern structures of the magnetopause boundary layer change
with the direction of the interplanetary magnetic field (IMF).
The magnetopause has been more intensively studied when
the IMF is southward, and it is found that its structures are
affected mainly by the reconnection and related phenomena
[Paschmann, 1979; Paschmann et al., 1979; Sonnerup et al.,
1981; Mitchell et al., 1987; Gosling et al., 1990, 1991].
When the IMF is northward, the magnetic field transition has
been found to be still a rotational discontinuity [Paschmann
et al., 1990]. The plasma transition consists of multiple
layers with relatively uniform structure inside each layer
[Song et al., 1990b].

The structure of the magnetopause boundary layer depends
upon more than just the IMF direction. It also depends upon
the magnetic field strength and the ratio of plasma thermal
pressure to the magnetic pressure, or beta. Observations of
accelerated flow events have shown that the magnetic
pressure has to be strong relative to the plasma pressure for
reconnection [Paschmann et al., 1986; Scurry et al., 1994].
Statistically, the low solar wind Mach number is associated
with large magnetic field strength and low plasma beta in the
solar wind. Furthermore, low plasma beta in the shocked
solar wind, or the magnetosheath, requires low Mach number
upstream of the bow shock. Under the low Mach number
condition the magnetic field plays a dominant role in any
reconnection related physical processes occurring at the
magnetopause boundary layer. We expect the effects of
magnetic reconnection to be strongest, whether the
reconnection occurs at high latitude for northward IMF or on
the dayside for southward IMF. Reconnection should be
least important when the solar wind plasma is weakly
magnetized under conditions of very high Mach number and
high plasma beta.

The dependence of the structure of the magnetopause upon
the Mach number has not been investigated systematically.
Several authors studied the structures of the magnetopause
for northward IMF. Song et al. [1990b] reported a detailed
case study of the magnetopause for northward IMF. In this
case the magnetosonic Mach number was 6, a typical value
at 1 AU. Paschmann et al. [1993] reported a survey of 22
magnetopause crossings for low local magnetic shear at the
magnetopause. To determine the magnetic field effects, we
examine in detail a limited number of cases under extreme
conditions when the IMF and the magnetosheath field are
strong rather than a large number of more typical conditions.
The purpose of this paper is to document special features of
magnetopause structures for low Mach number and strongly
northward IMF in a preliminary study of the Mach number
dependence of the magnetopause boundary layer. We discuss
common features observed at both typical and low Mach
numbers. We emphasize special features observed under low
Mach number condition.

In this paper we present a detailed case study of two
magnetopause crossings observed by the dual ISEE
spacecraft under conditions of low Mach number and
strongly northward IMF. The magnetosonic Mach numbers
in the two cases presented in the next section are 4.2 and 3.1,
respectively. The statistical survey of the solar wind
parameters of the entire solar cycle 21 at 1 AU indicated that
the magnetosonic Mach number is greater than 4 more than
75% of time and 3.5 more than 90% of time [Luhmann et
al., 1993]. Thus, the Mach numbers in this study occur
infrequently but should not be considered rare.

2. Observations

The data used in this study include those from the ISEE
1 and 2 magnetometer [Russell, 1978] and ISEE 2 Fast
Plasma Experiment (FPE) [Bame et al., 1979] for the
magnetopause crossing and from ISEE 3 and IMP-8 for
upstream solar wind. The first case, the outbound crossing of
ISEE 1 and 2 orbit 299 on October 5, 1979, is presented in
section 2.1 and the second case, the inbound crossing of
ISEE 1 and 2 orbit 314 on November 11, 1979, is presented
in section 2.2. In both cases the solar wind is very steady.
The average properties of the solar wind parameters near the
magnetopause crossings are listed in Table 1. The ISEE 1
and 2 trajectories during these intervals projected on to the
GSM XY and YZ planes are shown in Figure 1. Also shown
in Figure 1 is the average magnetopause position in the XY
plane.

Table 1. Solar Wind Parameters

October 5, 1979

November 11, 1979

Magnetic field in GSM (nT)

(4.3, -1.2, 6.7)

(9.34, -9.64, 11.70)

Magnetic field strength (nT)

8.0

18.0

Density (cm^-3)

5.5

10.1

Velocity (km/s)

342

411

Ion temperature (K)

1.7E4

9.5E4

Plasma beta

0.2

0.2

Magnetosonic Mach number

4.2

3.1

2.1. October 5, 1979

Figure 2 shows the ISEE 2 data during the outbound
magnetopause crossing on October 5, 1979. The top panel
shows the magnetic field in GSM coordinates and the field
strength with time a resolution of 4 s, which are averaged
from the full resolution data (0.25 s) with an overlapped
window of 12 s. The lower panels show ion density,
temperature, ion beta, and velocity with a time resolution of
12 s. They are deduced from the two-dimensional FPE
measurements. The two electrostatic analyzers of FPE in the
ecliptic plane produces complete two-dimensional ion and
electron distributions measurement within 55o elevation
angle every spacecraft spin of 3 s. Highlighted by shading is
an interval from 0217 to 0247 UT for the magnetopause
boundary layer crossing, which will be discussed in detail.
The spacecraft position in GSM is (9.61, -3.34, 5.04) RE at
0230 UT. The simultaneous observations of the IMF in
GSM, the solar wind density, velocity, temperature, and
dynamic pressure are shown on Figure 3 from both the ISEE
3 and IMP 8 since neither of them has complete data
coverage during this interval. In Figure 3, the ISEE 3 data
are lagged -70 min, which is the estimated time delay from
ISEE 3 to the Earth's magnetosphere. The IMF is very
steady and strongly northward during the interval. The clock
angle of the IMF is 10o from the due northward direction.
The magnetosonic Mach number is 4.2. The solar wind
plasma beta is 0.2.

From Figure 2 it is very difficult to identify the
magnetopause crossing from the magnetic field data alone.
The magnetic field on both sides of the magnetopause has
roughly the same strength and there is little magnetic shear
across the magnetopause since the IMF is strongly
northward. From the FPE plasma data it is clear that there
exists a boundary layer (shaded area) where the average
plasma properties are intermediate between their
magnetospheric and magnetosheath values. We note that the
plasma beta in the magnetosheath is very low and
comparable to the beta in the magnetosphere. The structure
of the boundary layer is shown more clearly in Figure 4 from
0215 to 0253 UT. In the upper four panels the high-
resolution (0.25 s) magnetic field data are displayed in the
boundary normal coordinate system, in which L is in the
direction of the magnetospheric field and N is normal to the
local magnetopause boundary. The next three panels show
ion density, temperature and velocity from two-dimensional
FPE data. In the bottom panel of Figure 4 is shown the
north-south component of the ion flow velocity (Vz) with a
time resolution of 50 s. The Vz component is deduced from
three-dimensional FPE measurements at a slower rate than
the two-dimensional measurements.

Depletion layer: Immediately outside the magnetosphere
in the magnetosheath there is a strong density depletion layer
( 0247 - 0315 UT). The inner edge of the depletion layer is
the magnetopause, marked by a dashed line at 0247 UT in
Figure 2 and Figure 4. This density depletion has been observed as
a common feature in the magnetosheath for northward IMF
and typical Mach numbers at 1 AU [e.g., Paschmann et al.,
1978, 1993; Song et al., 1990a, b; Anderson et al., 1991;
Phan et al., 1994]. There are strong Pc 1 waves ( 1 Hz) on
the magnetosheath field lines. These waves are mainly
transverse and appear mainly in the BM and BN components.
This is also similar to the typical Mach number cases. For a
typical Mach number case in the work by Song et al. [1990b]
these waves are found to be generated in the density
depletion layer.

Boundary layer: The boundary layer occurs from 0217
to 0247 UT, with the outermost boundary at the end of the
density depletion. The innermost boundary is not well
defined since there is no sharp transition to the
magnetosphere. The density and temperature gradually reach
their magnetospheric values near 0217 UT.

From the density and temperature the plasmas are
magnetosheathlike on average in the outer part of the
boundary layer ( 0226 - 0247 UT). The inner boundary layer
( 0217 - 0126 UT) is dominated by the magnetospheric
particles, as evidenced by the higher temperature. The Pc 1
waves, which are very strong in the depletion layer, are also
present in the outer boundary layer with reduced intensity,
but are absent in the inner boundary layer (prior to 0226:37
UT as marked by a solid line in Figure 4). These properties
are same as those reported by Song et al. [1990b] in
boundary layers under typical Mach number condition. One
difference is that the outer and inner boundary layers are not
separated by a sharp transition in ion temperature in the low
Mach number case.

Pulses and accelerated flows in the boundary layer:
The special feature in this low Mach number case is that
there is much structure inside the boundary layer as shown
in Figure 4. The most striking characteristic is a series of
pulses in field strength, density, velocity, and temperature.
The pulses in the magnetic field occur only in the BL
component and are compressional. These pulses are thus
associated with the changes in field strength, but not field
direction.

The Pc 1 waves are occasionally absent in the boundary
layer (shaded intervals 1, 2, 3, and 4 in Figure 4). More
detail can be seen in Figure 5. The upper two panels of
Figure 5 show the high-pass filtered BM and BN components,
respectively. The low cut off frequency for the high-pass
filter is 0.1 Hz, and the Nyquist frequency is 2 Hz. The
bottom panel of Figure 5 shows the magnetic field
magnitude. The drops of Pc 1 wave amplitude coincide with
pulses of field strength depressions and higher temperature
plasmas (shaded interval intervals 1, 2, 3, and 4 in Figure 4).
The properties of the waves are controlled by the plasma
conditions. Changes in the wave pattern indicate changes in
the plasma distribution function. At the magnetopause, such
drastic changes may indicate changes in the field topology.
Song et al. [1990b] have shown the correlation between the
Pc 1 wave and the field topology. The decrease of Pc 1 wave
amplitude in this case is consistent with the boundary layer
being on closed field lines during these intervals. Similarly
these waves are completely absent in the inner boundary
layer and the magnetosphere. However, the energy
spectrograms of ISEE 2 FPE ions (not shown) do not suggest
a mixture of magnetospheric and magnetosheath plasma in
these pulses. Rather, this plasma looks like heated sheath
plasma with no hot magnetospheric plasma component
present on the energy spectrograms.

This argument is supported by the two-dimensional ion
and electron distribution functions observed inside these
pulses. Figure 6 shows ISEE 2 FPE 3-s snapshots of ion (top
panels) and electron (bottom panels) distribution functions at
times marked by solid lines in Figure 5, all
within the high-density and high-temperature pulses. In the ion distribution
functions there are no hot ions from the terrestrial ring
current which have energies corresponding to speeds well
above 800 km/s, nor transmitted magnetosheath ions which
would appear as a cooler and strongly flowing ions in the XY
plane. The observation of electron distribution functions leads
to the same conclusion. Thus the particles in these pulses are
unlikely to be the hot magnetospheric plasmas entering the
boundary layer locally, nor local magnetosheath plasma
heated while crossing the magnetopause since there is little
magnetic shear, nor a mixture of both. The observations
suggest these pulses are heated magnetosheath plasma that
entered from elsewhere.

For the rest of the time (unshaded intervals 1', 2', 3', and
4' in Figure 4), the particles are dominated by the cool
magnetosheath plasma. Figure 7 shows 3-s snapshots of ion
(top panels) and electron (bottom panels) distribution
functions at times marked by dashed lines in Figure 5, all
outside the high-density and high-temperature pulses. Only
the cool sheath plasma component is observed in these
density contours. The presence of Pc 1 waves suggests that
the boundary layer is on open field lines where large
temperature anisotropics have been reported [Song et al.,
1993; Anderson et al., 1994]. However, the flows are
accelerated to velocities well above the magnetosheath values
during the unshaded intervals 1', 2', and 3', or near the inner
edge of the boundary layer as shown in Figure 4. From the
bottom panel of Figure 4, it is evident that the high speeds
have very strong Vz component, that is roughly a
field-aligned component. The positive sign of the Vz peak
indicates the accelerated plasmas flow northward.

Scale lengths: We have also examined the simultaneous
data from the dual ISEE 1 and 2 magnetometers to separate
the temporal variation from the spatial variation and to
estimate the velocity and the spatial extent of the boundary
layers. During the time interval of the magnetopause
boundary crossing, ISEE 1 and 2 travel outbound in an
identical trajectory (see Figure 1) but ISEE 2 leads ISEE 1
about 11 min. The spacecraft velocity is 2.47 km/s along the
orbit and 2.11 km/s along the boundary normal direction.
Since the pulses in the magnetic field occur mainly in the
field-aligned direction, we show simultaneous observations
of only the magnetic field strength in Figure 8. The top
panel of Figure 8 shows time series of the magnetic field
strength with the highest time resolution. Both spacecraft
observe the impulsive structures. ISEE 2 observes them
earlier than ISEE 1 does. The time delay of the field
observations is approximately the same as their orbital time
delay, or 11 min. This observation suggests that the boundary
layer as a whole is nearly stationary in space and the process
that causes the pulses persist over a long period of time. The
pulses in the boundary layer are substructures and each
individual pulse may be a temporal structure inside the
boundary layer. This can been seen clearly from the bottom
panel of Figure 8, in which we show the magnetic field
strength versus the spacecraft position X. Both ISEE 1 and 2
observe the pulses in nearly the same spatial range, while the
observations are nearly 11 min apart. The average velocity of
the boundary motion as a whole can be estimated by using
the first pulses as the timing of the inner edges of the
boundary layer. This velocity is extremely small, 0.07 km/s
as calculated from the time delay (664 s) and the spatial
separation of the first pulse observed by ISEE 1 and 2 (48.57
km along the boundary normal direction).

We also note in Figure 8 that at one point from 0245 to
0246 UT, the outermost spacecraft, ISEE 2, is in a heated
plasma pulse and the innermost spacecraft, ISEE 1, is in the
sheath-like plasma. This argues strongly that the pulses are
not caused by boundary motion and is consistent with the
extremely small boundary velocity obtained above.

Since the spatial extent of the boundary layer as a whole
appears to be nearly stationary, the observed layers are
mainly spatial variations due to the motion of the spacecraft
for this particular case. We can estimate the spatial scale of
these layers from the velocity of the spacecraft relative to the
boundary layer (2.11 km/s along the boundary normal
direction) and ignore the velocity of the boundary layer
motion as a whole. The density depletion layer in the
magnetosheath is observed from 0247 to 0315 UT
immediately outside the magnetopause boundary layer. Thus
the thickness of the depletion layer is 3545 km. From the
time rate of change of density we can deduce that the density
varies at a rate of 0.0051 cm-3 per second in the depletion
layer. This gives a density gradient of 0.0024 cm-3/km in the
depletion layer. The duration of the boundary layer is 35
min. The thickness of the boundary layer is 4430 km, or 0.7
RE. The pulses in the boundary layer appear to be temporal
structures since pulses at ISEE 1 do not have one-to-one
correlation to those at ISEE 2 as you can see from the
bottom panel of Figure 8. The two spacecraft observations
are unable to resolve their spatial structures.

2.2. November 11, 1979

Figure 9 shows the ISEE 2 data during the inbound
magnetopause crossing from 2130 to 2330 UT on November
11, 1979. Similar to Figure 2, the top panel is the time series
of the magnetic field in GSM coordinates with a time
resolution of 4 s. The bottom panels show ion density,
temperature, plasma beta, and velocity with time resolution
of 12 s. The spacecraft trajectory during this interval is
shown in Figure 1. At 2230 UT, the ISEE 2 position in GSM
coordinates is (8.58, -0.63, -1.96) RE. The spacecraft is very
close to the subsolar point. The ISEE 3 solar wind data for
this interval are shown in Figure 10. The estimated time
delay from ISEE 3 to the Earth's magnetosphere is about 50
min. The average solar wind parameters corresponding to the
magnetopause boundary crossing are listed in Table 1.
During this interval, the IMF is also strongly northward with
clock angle of 39o from the due northward direction. The
magnetosonic Mach number is 3.1 and the solar wind plasma
beta is 0.2.

The ISEE 2 data in Figure 9 have many features similar
to the October 5, 1979, case. The magnetic shear is very
small due to the strong IMF strength and northward Bz
component. The plasma beta is very low throughout this
interval. The magnetopause crossing is not obvious in the
lower resolution magnetic field data but can be see clearly
from the two-dimensional FPE plasma data. There exists a
boundary layer, highlighted by shading, in which the plasma
properties are intermediate between their magnetospheric and
magnetosheath values. Figure 11 shows details of the
boundary layer from 2220 to 2300 UT using the highest time
resolution magnetic field data. The magnetic field data are
displayed in the boundary normal coordinate system. The
solid line at 2225 UT marks the entry into the boundary
layer, that is, the crossing of the magnetopause. We can note
the following points from Figure 9 and Figure 11:

Depletion layer: In the magnetosheath immediately
outside the boundary layer, there is a strong density depletion
layer, where the density decreases gradually as the
magnetopause is approached. Pc 1 waves are also present in
the depletion layer. These properties are similar to the
previous case and common for northward IMF regardless of
the Mach number. Since the spacecraft is very close to the
subsolar stagnation point, the flow velocity in the XY plane
decreases to nearly zero when the spacecraft gets close to the
magnetopause. The density depletion layer ends at the
magnetopause at 2225 UT (the solid lines in Figure 9 and
Figure 11).

Boundary layer: The boundary layer is observed from
2225 to 2252 UT. The transition from the magnetosheath to
the boundary layer is sharp, marked by a sudden increases in
temperature and a flat density profile. The intensity of the
high-frequency waves decreases upon entry into the
boundary layer, and eventually the waves disappear. The
outer part of the boundary layer is quite uniform except for
an isolated pulse of enhanced temperature and decreased
density near 2241 UT. Otherwise, the ion density and
temperature stay at a plateau level. There is no sharp
transition from the boundary layer to the magnetosphere
proper. Similar to the previous case, the density and
temperature gradually approach their magnetospheric values,
although the transition at 0252 UT is quite sharp. These
properties are generally consistent with those observed for
typical Mach numbers.

Accelerated flow in the boundary layer: The special
feature in this low Mach number case is the high speed flow
earthward of the magnetopause in the boundary layer. The
ions are significantly accelerated at the outer edge of the
boundary layer, associated with the sharp increase of the ion
temperature. The accelerated flow velocity has strong +Vx
and +Vz components. Combining with the magnetic field
direction, we find that the accelerated plasma flows
northward and mainly tangent to the magnetopause and
field-aligned. The maximum field-aligned flow velocity
reaches about 155 km/s northward. Sunward of the
magnetopause, the same velocity component is about 60 km/s
southward. At this time the spacecraft is located about 2 RE
south of the magnetospheric equator, thus the accelerated
flow is from the high latitude region to the equator. These
observations strongly suggest that the ions in the high speed
flow have not entered and accelerated locally across the
magnetopause due to the lack of magnetic shear at the
magnetopause. This can be verified by examining the
condition of tangential momentum balance between plasma
and magnetic field [e.g., Paschmann et al., 1979; Sonnerup
et al., 1981]. If reconnection were occurring locally the
plasma would have been accelerated by the j B force as it
crossed the magnetopause current layer. For the November
11, 1979, event the tangential velocity jump across the
magnetopause is about 215 km/s at its maximum. The shear
of tangential magnetic field across the magnetopause current
is about 4o and tangential field jump is -16.3 nT. Using the
observed proton density of 8 cm-3 and about 10% alpha ion
content (from solar wind data), the tangential velocity jump
was estimated to be about 110 km/s according to tangential
momentum balance. Such a jump can only provide a 50 km/s
field aligned velocity earthward of magnetopause, which was
much smaller than the observed field-aligned velocity. Thus
we believe the reconnection occurs remotely in this case.

Scale lengths: The combination of ISEE 1 and 2 magnetic
field data are used to calculate the velocity of the
magnetopause motion. For this interval, ISEE 1 and 2 travel
inbound in an identical trajectory and ISEE 2 leads ISEE 1
by about 23 min. The spacecraft velocity is 3.05 km/s along
the orbit and 2.05 km/s along the boundary normal direction.
Figure 12 shows the simultaneous observation of the
magnetic field strength from ISEE 1 and 2. The top panel of
Figure 12 shows the time series of the field strength and the
bottom panel shows the field strength versus the X position
of the spacecraft. There is a decrease of the field strength, a
signature of the boundary layer entry, in both the ISEE 1 and
2 data. However, ISEE 2 enters the boundary layer 1742 s
earlier than ISEE 1 does. The separation of the spacecraft
position at the boundary layer entry (2254:30 UT for ISEE
1 and 2225:28 for ISEE 2) is 785 km along the boundary
normal direction. Thus the velocity of the magnetopause
motion is 0.45 km/s, and the magnetopause motion is inward.

From the speed of the boundary we find that the velocity
of spacecraft relative to the magnetopause is 2.50 km/s along
the boundary normal direction. From the plasma time series
we find that the thickness of the depletion layer is 2700 km
and the density gradient is 0.0056 cm^-3/km. The boundary
layer thickness is 4500 km, or 0.7 Re.

3. Summary and Discussion

We have presented above observations of the structure of
the magnetopause boundary layer under low Mach number
and northward IMF conditions. There are many features that
are common for the northward IMF regardless of Mach
number, including: (1) a nearly stationary boundary layer
with a thickness 0.7 RE and an average speed < 1 km/s; (2)
a strong density depletion layer immediately outside the
magnetopause with a thickness 3000 km and a density
gradient of the order of a few 10^-3 cm^-3/km; (3) an overall
flat density profile in the boundary layer; and (4) no sharp
transition from the boundary layer to the magnetosphere.

However, we have seen some properties in the low Mach
number cases that have not been reported in previous study
under typical Mach number. They are (1) at least in one case
(October 5, 1979) impulsive structures are present in the
boundary layer and the process that causes them persist over
a long time period; (2) the pulses contain mainly heated
magnetosheath plasma entered elsewhere without locally
transmitted magnetosheath plasma and hot magnetospheric
plasma components; and (3) there is also clear evidence of
accelerated flows inside the boundary layer in both cases.
These accelerated flows have a strong Vz (field-aligned)
component, indicating that they come from high-latitude
region.

These observations support the idea that direct plasma
entry from the magnetosheath via diffusive processes is not
a major contributor to the plasma population inside the
boundary layer. The overall density is quite constant within
the boundary layer, that is, the density gradient expected
from diffusive entry is absent. The special features observed
in the low mach number cases provide additional evidence
for the nonlocal entry of the plasma inside the boundary.
First, during the high-density and high-temperature pulses in
the October 5, 1979, case, the plasma distribution functions
indicated that these pulses contain neither hot magnetospheric
plasma nor the transmitted magnetosheath plasma. They are
heated magnetosheath plasmas that entered the boundary
layer from elsewhere. Second, the accelerated flows are
observed in spite of lack of local field shear for both cases.
The flows have a strong field-aligned component. Diffusive
processes can not explain the plasma accelerations observed
within the boundary.

An alternative explanation for the boundary layer structure
is reconnection above the cusps for northward IMF [e.g.,
Dungey, 1961; Crooker, 1979; Cowley, 1981; Paschmann et
al., 1990; Gosling et al., 1991; Song and Russell, 1992], and
observations in this study support this idea. In the
observation of October 5, 1979, crossing the boundary layer
consists of several pulses of heated sheath plasma.
High-frequency waves near 1 Hz are present for most of the
boundary layer but are absent during these pulses. These
waves, common for northward IMF, are found to be
generated and maintained in the density depletion layer and
do not propagate into the magnetosphere [Song et al.,
1990b]. It might be caused by the fact that the magnetic field
lines are alternately connected to the magnetosphere during
these pulses and to the solar wind for the rest of the time
since the change of plasma distribution function might be
expected at the boundary of open and closed field lines. We
note that the presence and absence of 1 Hz waves due to the
change of plasma distribution function maybe an indication
of change of field topology. Although it may not be an
unambiguous measurement of the boundary between open
and close field lines, it is a very important problem and
needs further investigation.

The observations of the plasmas during these pulses are
consistent with the reconnection picture. On the field lines
that are apparently connected to the solar wind (open field
lines), the plasmas are basically sheathlike with reduced
density. However, the pulses of plasmas on closed field lines
look like heated sheath plasma with no hot magnetospheric
plasma component and have densities much higher than those
adjacent to them. Thus they are clearly not a mixture of
magnetospheric and magnetosheath plasmas from adjacent
regions. Rather, they could be magnetosheath plasmas which
originate and are heated in cusp regions due to high-latitude
reconnection. The accelerated flow on open field lines may
also be the results of the reconnection at high latitude.

As the solar wind plasma enters the magnetosphere from
the reconnection site, the j BN force will accelerate the
plasma flow in directions tangent to the magnetopause
boundary and away from the reconnection site due to the
tangent momentum balance. For the northward IMF geometry
when the reconnection occurs poleward of cusp region (either
northern cusp or southern cusp), the accelerated plasma
streams both tailward and equatorward from the cusp
reconnection site. Thus, accelerated flow can move from the
cusp reconnection site to the observation site near the equator
for both northern and southern cusp reconnection. In both
cases studied the accelerated flow velocity has a strong
positive Vz peak (northward), indicating that the reconnection
occurs above southern cusp region.

4. Conclusions

We have used ISEE magnetometer and FPE data to
examine the structure of the magnetopause boundary layer
under conditions of low Mach number and strongly
northward IMF. When the solar wind Mach number is low,
the IMF and magnetosheath field strength are large, and we
expect effects of reconnection to be the strongest. In this
study we have observed some structures that are common for
both typical and low Mach numbers when the IMF is
northward. However, we find some structures in the
boundary layer that occur under low Mach number when the
IMF is northward. These include impulsive structures in the
boundary layer, nonlocal entry of heated magnetosheath
plasma into the boundary layer, and clear evidence of
accelerated flow for strongly northward IMF. Reconnection
over the cusp regions can explain the observed field, plasma
and flow signatures. Thus, we conclude that reconnection is
important in the formation of the boundary layer for
northward IMF at low Mach number, as well as for
southward IMF.

Acknowledgments. The work at UCLA was supported by the
National Science Foundation under research grant
ATM91-11913. The work at Los Alamos National Laboratory
was performed under the auspices of the U.S. Department of
Energy and supported in part by NASA.

The Editor thanks R. L. Kaufmann and another referee for
their assistance in evaluating this paper.

Figure 4. High-resolution magnetic field data and FPE ion data showing the structure of boundary layer
for the October 5, 1979, magnetopause crossing. The bottom panel is the Vz component (north-south) from
three-dimensional FPE measurements at a lower rate.

Figure 5. High-pass-filtered magnetic BM and BN components and the magnetic field strength for the
October 5, 1979, magnetopause crossing.

Figure 6. Three second snapshots of two-dimensional (top) ion and (bottom) electron velocity distribution
function observed inside the high-density and high-temperature pulses for the October 5, 1979, case. The
corresponding times are marked by solid lines in Figure 5. The distributions are shown as contours of
constant phase-space density spaced logarithmically. The numbers on dotted circles is the velocity scale in
kilometers per second.

Figure 7. Three second snapshots of two-dimensional (top) ion and (bottom) electron velocity distribution
function observed outside the high density and high temperature pulses for the October 5, 1979, case. The
corresponding times are marked by dashed lines in Figure 5.

Figure 8. (Upper) Time series of magnetic field strength from both ISEE 1 and ISEE 2. (Lower) The
magnetic field strength from both ISEE 1 and ISEE 2 as a function of the spacecraft X coordinate.